The transport of a relativistic electron beam was studied experimentally and numerically in a magnetic field configuration which consists of a focusing wiggler as well as an additional axial guide field. A new beam divergence was found for the lower side of the magnetoresonance. It was identified as due to the effect of the second harmonic mode of the cyclotron frequency in the resonance, determined by /jft i i -A^rn for n =2. The harmonic resonance was detected for the planar wiggler first, and it could appear also for the helical one. PACS numbers: 41.75.Ht, 41.85.Ja, 52.75.MsSuccessful operation of a Raman free electron laser (FEL) using a relativistic electron beam (REB) has been reported for both planar and helical wiggler configurations [1][2][3][4]. To obtain a high-power performance of the FEL, a stable transport of the beam in the wiggler is one of the key issues. The introduction of an axial uniform guide field superimposed on the wiggler is usually required to keep the orbital stability in the transport, while it will involve some other complex features like the magnetoresonance between the cyclotron motions and the wiggling of the electrons [5]. It is well recognized that the beam diverges and spills out when the electron cyclotron frequency flu in the guide field B g approaches the wiggling frequency K w v\\, where k w is a wiggler wave number, Clw^eBg/yorrie, yo = {l "~ (v 2 /c 2 )} _1 *, v and c are electron and light speed, respectively, and the suffix II indicates axial components [6].In this Letter we describe an experimental and numerical study on the transport of a REB (£6~~0.8 MeV and /6~300 A) through a combination of a focusing type planar wiggler [7] and an axial field: We retain a relatively low additional B g to control the beam dynamics at the entrance of the wiggler and the FEL performance. We found a new type of beam loss at an axial field far lower than the value of B g corresponding to the fundamental magnetoresonance flii^&wm; e.g., the loss is in the parameter region of the so-called group I. Therefore it clearly differs from the "harmonic" gyroresonance which was predicted to occur in group II at flu ^nk w v\\ (n an integer) by Chu and Lin [8], where ft H is a timeaveraged cyclotron frequency in B\\. We need another explanation for the new resonance. We made an analytical study and a numerical simulation on the resonance and its effect on the beam transport through the wiggler. We carried out a similar numerical study for the case of a helical wiggler and the result suggests we will also have the same kind of resonance and beam degradation. The magnetic field in a planar focusing wiggler with a uniform field B g G z is given by B(r) = s

A mm wave amplifier experiment on a free electron laser was performed using an intense, mildly relativistic electron beam (0.7–0.9 MeV) with a combination of a focusing planar wiggler and a weak additional axial magnetic field in the so-called ‘‘group 1’’ region. A beam transport ratio of over 80%, through the whole wiggler length, was realized. In an amplification experiment, a spatial growth rate of 56 dB/m was obtained at a frequency of 45 GHz, and a power saturation was observed at a level of 6 MW, where the total gain was estimated to be 52 dB. It was shown that the off-axis component of the beam contributes considerably to the rf amplification in the focusing wiggler.

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Arrays of microtriodes have recently become available due to the development of microfabricated field-emission electron sources. Computer simulation has shown that the brightness of beams emitted by them is significantly higher than that of the common microbeams, and possible application of the accelerated beam to free electron lasers has been discussed. Experimentation on beam generation has started, but methods for diagnosing the beam have not yet been established. Difficulty is predicted, because of the high brightness, in applying the conventional methods of emittance measurement. In this paper we propose a new method that determines the emittance without using apertures. The cross section of a converging beam is elongated by a quadrupole lens, and parameters of the emittance ellipse are obtained from the beam size on a screen when changing either the strength or the axial position of the quadrupole lens.

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